22968 | Phys. Chem. Chem. Phys., 2014, 16, 22968--22973 This journal is© the Owner Societies 2014

Cite this:Phys.Chem.Chem.Phys.,

2014, 16, 22968

Barrierless tautomerization of Criegeeintermediates via acid catalysis†

Manoj Kumar,ab Daryle H. Busch,ab Bala Subramaniambc and Ward H. Thompson*ab

The tautomerization of Criegee intermediates via a 1,4 b-hydrogen

atom transfer to yield a vinyl hydroperoxide has been examined in

the absence and presence of carboxylic acids. Electronic structure

calculations indicate that the organic acids catalyze the tauto-

merization reaction to such an extent that it becomes a barrierless

process. In contrast, water produces only a nominal catalytic effect.

Since organic acids are present in parts-per-billion concentrations in

the troposphere, the present results suggest that the acid-catalyzed

tautomerization, which can also result in formation of hydroxyl

radicals, may be a significant pathway for Criegee intermediates.

The direct generation, detection, and reaction of the Criegeeintermediate in the gas-phase by Taatjes and coworkers1,2 hassparked a renewed interest in Criegee chemistry. The Criegeeintermediates are carbonyl oxides that are principally producedin olefin ozonolysis.3 In the troposphere, they play a significantrole in chemistry involving secondary organic aerosols, acids,peroxides, sulfates, and nitrates. At the same time, Criegeechemistry is a key part of ozonolysis-based syntheses that offer safeand scalable routes for preparing pharmaceutical intermediatesand other value-added chemicals.4–6 The Criegee intermediates arealso implicated in the reaction cycles of flavin-dependent Baeyer–Villiger monooxygenases7 that provide a green route for synthe-sizing enantiopure drug compounds.8

Recently, Welz et al. reported a study of the kinetics for thereactions of Criegee intermediates (CH2OO, anti-CH3CHOO,and syn-CH3CHOO) with simple carboxylic acids (HCOOH andCH3COOH).9 The measured rate coefficients were on the order

of 10�10 cm3 molecule�1 s�1, orders-of-magnitude larger thanthose previously estimated based on ozonolysis experiments10

and atmospheric modeling studies.11 This indicates that thereactions of the Criegee intermediate with organic acids is a sub-stantially more important decay process than has been pre-viously thought. Moreover, it suggests that this pathway couldcompete with unimolecular decay and reaction with water as keyCriegee loss processes, especially in forested12 and urban environ-ments,13 where the acid levels are in the parts-per-billion range. Inthis letter we use electronic structure calculations to examine onepossible pathway for the reaction of carboxylic acids with Criegeeintermediates: tautomerization via a 1,4 b-hydrogen atom transferto form a vinyl hydroperoxide.

The uncatalyzed tautomerization of syn-CH3CHOO oxide ispredicted to have an activation barrier of B14.9–21.2 kcal mol�1

depending on the level of theory.14 However, the present calcula-tions find that this barrier is significantly lowered or even comple-tely removed in the presence of organic acids. In the troposphere,monocarboxylic acids and low molecular weight dicarboxylicacids, such as oxalic acid, are present in appreciable amounts(parts per billion, ppb)15–18 and their tendency to enhance gas-phasehydrogen-transfer reactions is increasingly being recognized;19–26

recent satellite measurements reveal that the formic acid (HCOOH)levels in urban air are much higher than previously estimated.27

It has even been suggested that sulfuric acid (H2SO4), an inorganicacid and a key precursor in aerosol formation, can catalyze suchhydrogen transfers.20,22,26 In addition, the role of aromatic acids tocatalyze particulate formation has been verified through laboratoryexperiments.28 Organic acids are also produced during the variousunimolecular and bimolecular Criegee processes and thus can playa role in ozonolysis-based syntheses.3

Hydroxyl radical (�OH) plays a central role in the oxidationchemistry of the troposphere.29 Its sulfuric acid-forming reac-tion with sulfur dioxide1 deeply impacts aerosol formation andthe earth’s climate.30 While ozone photolysis is the primarysource of hydroxyl radical, a growing body of experimental31–37

and theoretical evidence37–43 indicates that Criegee chemistryalso serves as an important source of tropospheric �OH.

a Department of Chemistry, University of Kansas, Lawrence 66045, USA.

E-mail: wthompson@ku.edub Center for Environmentally Beneficial Catalysis, 1501 Wakarusa Drive, Lawrence,

KS 66047, USAc Department of Chemical and Petroleum Engineering, University of Kansas,

Lawrence, KS 66045, USA

† Electronic supplementary information (ESI) available: Computational details,structures of Criegee intermediates considered, calculated free-energy reactionprofiles, and tables containing detailed energetic information. See DOI: 10.1039/c4cp03065f

Received 11th July 2014,Accepted 19th September 2014

DOI: 10.1039/c4cp03065f

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Specifically, the tautomerization of the Criegee intermediate toform the hydroperoxide olefins constitutes the main mecha-nism for �OH generation via ozonolysis.34,38,39,41 As shown inScheme 1, this reaction involves a 1,4 b-hydrogen atom transfer(H-transfer) between the b-carbon of the Criegee intermediateand its terminal oxygen, and the subsequent homolytic O–Obond cleavage to form �OH. The possibility of secondary�OH-forming Criegee channels has also been discussed in theliterature.40,44,45 The impacts outlined above for facile tauto-merization of Criegee intermediates, including the potentialformation of �OH, provides a strong impetus to study the role ofacid catalysis in Criegee chemistry.

In this letter, the results of electronic structure calculationson the unassisted and organic acid-assisted intramolecular 1,4hydrogen-transfer in Criegee intermediates with b-hydrogensare presented. Free energy calculations predict that an acid-assisted reaction to form a vinyl hydroperoxide is a barrierlessprocess irrespective of the Criegee intermediate considered. Bycomparison, water has only a weak catalytic effect on the free-energy barrier, in part due to entropic factors, suggesting that the1,4 hydrogen-transfer may be relatively insensitive to humidity.

While there are ample examples of a single molecule or amolecular cluster reducing the activation energy of an atmo-spherically or industrially important reaction, to our knowledgethe catalytic effect of acids on the tautomerization of a Criegeeintermediate has not been previously examined. To explore thereaction, we examined the uncatalyzed and catalyzed hydrogen-transfer in C2- and C3-based Criegee intermediates as well as inthose produced during the ozonolysis of isoprene and a-pinene(Scheme S1, ESI†). Note that such a reaction mechanism cannotoccur with the simplest Criegee intermediate, CH2OO, because itlacks b-hydrogens. Isoprene, the most common biogenic hydro-carbon in the troposphere, is emitted by terrestrial vegetation.46

It dominates the global total emission of biogenic volatile organiccompounds with its annual emission strength varying between200 and 600 teragrams of carbon. Among the monoterpenes that

account for 10–50% of the total emitted mass of non-methanebiogenic volatile organic compounds, a-pinene has the highestemission rate into the atmosphere and is believed to be the largestcontributor to the formation of secondary organic aerosols.47–50

Electronic structure calculations (the details of which areprovided in the ESI†) are performed at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory (298.15 K, 1 atm) thathas been previously found to describe hydrogen-transfer pro-cesses,19,20 including Criegee chemistry,51 well. The accuracy ofthe M06-2X/aug-cc-pVTZ theoretical method chosen for geo-metry optimization is confirmed by comparing the thermo-chemistry of the uncatalyzed reaction calculated using variousDFT and wavefunction based methods including CCSD, CCSD(T),and LPNO-CEPA/1 (Tables S1–S3, ESI†).

The intramolecular 1,4 hydrogen-transfer in a Criegee inter-mediate with b-hydrogens is a modified version of keto–enoltautomerism in which a b-hydrogen shifts to the terminalCriegee oxygen leading to the formation of vinyl hydroperoxide,indicated as the enol form in Scheme 1. In the absence of anycatalyst, the reaction follows a concerted mechanism through a5-membered cyclic transition state. At the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level, the tautomerization of syn-CH3CHOOhas a calculated free-energy barrier of DG‡ = 16.7 kcal mol�1

(Fig. S1, ESI†), in agreement with previous theoretical estimatesof 14.9–21.2 kcal mol�1.14 Moreover, the rearrangement is17.8 kcal mol�1 exoergic, which is also consistent with priorprojections of DE E �20.0 kcal mol�1. Interestingly, the tauto-merization is more favorable than conventional keto–enol pro-cesses, where DE‡ B 55 kcal mol�1.21,52 Moreover, the enol-likeform (vinyl hydroperoxide) is significantly lower in energy thanthe keto-like form (Criegee intermediate), whereas, the conven-tional enols tend to be 10–15 kcal mol�1 less stable than the ketoforms. This distinct feature of Criegee chemistry can be attributedto the zwitterionic nature of the Criegee intermediate’s electronicstructure that increases the nucleophilicity of the hydrogen-accepting terminal oxygen, thereby activating the cleavage ofthe C–H bond and favoring the formation of the enol-like form.

Because water is the dominant trace component in thetroposphere, its potential catalytic influence on the bimolecularCriegee chemistry has been repeatedly examined.14,40,41 Indeed,Anglada et al. have previously studied the water-catalyzed tauto-merization of C2- and C3-based Criegee intermediates as well asof those resulting from the isoprene ozonolysis.40 According totheir calculations, although water lowers the enthalpic barrier,the incorporation of entropic contributions raises the free-energybarrier to a nearly equivalent extent, drastically reducing theeffect of this reaction. They estimated that only 5.2–13.5% of thetotal bimolecular Criegee intermediate + H2O chemistry occursvia the tautomerization channel.

We have recalculated the reaction path for the water-catalyzed tautomerization to make comparisons with the acid-catalyzed reaction using the same theoretical description. Thecalculations are consistent with those of Anglada et al. in that awater molecule produces a significant change in the calculatedpotential energy surface (Fig. S2, ESI†), but the effect nearlydisappears upon inclusion of entropic effects, as shown in Fig. 1.40

Scheme 1 General mechanistic pathways for the unassisted (black), water-assisted (red), and acid-assisted (blue) tautomerization of a Criegee inter-mediate through 1,4 b-hydrogen transfer.

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Indeed, the calculated free-energy barrier of the water-catalyzedreaction is only 2.0 kcal mol�1 lower than for the uncatalyzedreaction.

On the other hand, in the presence of organic acids thetautomerization is not only enhanced, it becomes a barrierlessprocess. The HCOOH-catalyzed reaction proceeds through astrongly bound prereaction complex, Int1 (DG = �9.6 kcal mol�1),which involves two hydrogen bonds between HCOOH and theCriegee intermediate. Note that, as shown in Fig. 1, the O–H bondof HCOOH in Int1 is noticeably elongated due to the high nucleo-philicity of the terminal Criegee oxygen. Then, Int1 converts intoInt2, a hydrogen-bonded complex between HCOOH and vinylhydroperoxide, via a transition state that represents a submergedbarrier (relative to the separated reactants). The Int2 postreactioncomplex is 21.7 kcal mol�1 lower in free energy than the reactantsand, due to two intermolecular hydrogen bonds, 3.7 kcal mol�1

more stable than the separated products.Clearly, HCOOH causes a dramatic lowering of the barrier

relative to the unassisted and water-assisted tautomerizations.The calculated transition state for the HCOOH-catalyzed reactionlies 0.4 kcal mol�1 in free energy below the separated reactantsindicating a facile tautomerization of the Criegee intermediate.The catalytic superiority of a single HCOOH molecule is mainlyattributed to the pair of carbonyl and hydroxyl functionalitiesthat facilitate an efficient hydrogen transfer through a doublehydrogen-shift reaction. As a result, the distance across which

the b-hydrogen migrates is considerably reduced and the reac-tion occurs via a 9-membered cyclic transition state that is lesssterically strained than the transition states for the uncatalyzedand water-catalyzed reactions.

The cleavage of the O–OH bond in the vinyl hydroperoxideleads to the formation of �OH and vinyloxy radical. Contrary toearlier reports,41,45 recent multi-reference ab initio calculationsindicate that this bond breaking process involves a free-energybarrier of 18.8 kcal mol�1 and an endergonicity of 11.6 kcal mol�1.53

Since the vinyl hydroperoxide is formed with an excess free energy of18.0 kcal mol�1, the overall �OH forming reaction involving theacid-catalyzed Criegee tautomerization might be expected to be anearly barrierless reaction. However, the presence of the organicacid stabilizes the vinyl hydroperoxide through two hydrogen bondsand might delay or inhibit the �OH formation by accepting excessenergy. Additional study will be required to elucidate the role of theacid-catalyzed tautomerization on �OH formation, including inunderstanding the time or pressure-dependences seen in ozonolysisexperiments.54,55

The calculated free-energy profiles indicate that the HCOOH-catalyzed tautomerization may be an important decay channel forCriegee intermediates in the troposphere. Given the substantiallysubmerged barrier, it could be a competitive pathway with thebetter-studied, and also barrierless, addition reaction of HCOOHwith Criegee intermediates to form hydroperoxy alkyl esters.56,57

However, the recent experiments of Welz et al. on Criegeeintermediate reactions with formic acid do not show obviousindications of the tautomerization reaction, as the measuredrate constants are not strongly dependent on the presence of ab-hydrogen in the Criegee intermediate, i.e., the rate constants arethe same within error bars for anti-CH3CHOO + HCOOH and syn-CH3CHOO + HCOOH.9 However, the ionization energy of thetautomerization product of the latter reaction, vinyl hydro-peroxide, has been calculated to be 9.18 eV,2 sufficiently low thatit would presumably be detected in the experiments of Welz et al.(before it decays to form �OH). Thus, additional experimental andtheoretical work will likely be required to definitively determinethe role of the tautomerization reaction in Criegee intermediatereactions with carboxylic acids. The selective isotope labeling onthe methyl group of the syn-CH3CHOO (CD3CHOO) could be aninteresting experiment to test the proposed mechanism. Suchan isotopic substitution should exclusively lead to the formationof CD2CHOOH, rather than CD2CHOOD, and thus, the �OH, butnot the �OD should appear as the final product in the experiment.

Because dicarboxylic acids have an appreciable presence inthe troposphere,16,17 we next investigated their catalytic effecton Criegee intermediate tautomerization. We first examinedthe simplest dicarboxylic acid, oxalic acid. The catalyzed tauto-merization follows the same route as that involving HCOOHshown in Fig. 1 with only small differences in the relative freeenergies (Table S3, ESI†). The reaction catalyzed by malonicacid, the next larger dicarboxylic acid, has a higher barrier thanfor oxalic acid by 1.6 kcal mol�1. This effect is attributable to thepresence of a relatively weaker intramolecular hydrogen bondin the latter case;19 RO� � �H = 1.98 Å for oxalic acid compared toRO� � �H = 1.74 Å for malonic acid (Fig. S3, ESI†).

Fig. 1 CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ-calculated free-energyprofiles of HCOOH-catalyzed (green lines) and water-catalyzed (red lines)tautomerization of syn-CH3CHOO (298.15 K, 1 atm, kcal mol�1). The free-energy barrier of an uncatalyzed reaction is also shown (horizontal blackline). All Energies are reported with respect to the separated reactants. SeeFig. S2 (ESI†) for enthalpy profiles.

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It is also interesting to examine the role of the Criegeeintermediate structure by comparing the tautomerization ofthe syn-CH3CH2CHOO and (CH3)2COO isomers. As indicated inFig. 2, the calculated free-energy barriers of the unassistedtautomerizations of syn-CH3CH2CHOO and (CH3)2COO (see alsoFig. S4 and S6, ESI†) are similar to that of the syn-CH3CHOOreaction. This is consistent with a recent study by Liu et al., inwhich the ZPE-CCSDT/6-311+G(2d,p) calculated barrier heightsfor the uncatalyzed tautomerization of the C2 and C3 Criegeeintermediates were found to be similar.58 The current calcula-tions, presented in Fig. 2, find free-energy barriers of �1.2 and�3.4 kcal mol�1 for the HCOOH-catalyzed tautomerization ofsyn-CH3CH2CHOO and (CH3)2COO, respectively (see Fig. S5and S7, ESI†). This indicates there is a small, but non-negligible,dependence of the acid-catalyzed barrier on the Criegeeintermediate structure. Interestingly, the free-energy barriersof the water-catalyzed tautomerizations (which are just1.4–2.0 kcal mol�1 lower than the uncatalyzed reaction) differby only 0.5 kcal mol�1 and those for the uncatalyzed reactionsare only 0.1 kcal mol�1 apart.

The tautomerizations of two of the possible biogenic Criegeeintermediate conformers produced during the isoprene ozono-lysis (CIisoprene1 and CIisoprene2) is quite similar to that of syn-CH3CHOO. In particular, as indicated in Fig. 2, the two isoprenederived Criegee intermediates have similar uncatalyzed free-energybarriers, 17.2 and 16.9 kcal mol�1 for CIisoprene1 and CIisoprene2,respectively, compared to 16.7 kcal mol�1 for syn-CH3CHOO. Theparticipation of a water molecule also has a small effect on thisbarrier, reducing it by 1.6, 1.2, and 2.0 kcal mol�1 for CIisoprene1,CIisoprene2, and syn-CH3CHOO, respectively. In a recent study,40b

Anglada et al. have also stressed the importance of entropy effectsin the water-catalyzed tautomerization of CIisoprene1 and CIisoprene2,i.e., according to their calculations, the catalytic effect of a watermolecule on the tautomerization reaction is significantly reducedby entropic contributions. However, the results in Fig. 2 clearlyshow that formic acid has a dramatic catalytic effect, reducingthe free-energy barrier to 0.2 and �0.8 kcal mol�1 for CIisoprene1

and CIisoprene2 (see also Fig. S8–S11, ESI†), quite similar to theDG‡ = �0.4 kcal mol�1 barrier for syn-CH3CHOO.

Finally, we consider the tautomerization of one of the Criegeeintermediates formed during a-pinene ozonolysis (CIpinene).(Fig. S12–S13, ESI†). As indicated in Fig. 2, the calculatedfree-energy barriers for the uncatalyzed, water-catalyzed, andHCOOH-catalyzed tautomerization of the a-pinene Criegeeintermediate, CIpinene, are within 1.0 kcal mol�1 of the barriersestimated for the syn-CH3CHOO. Combined with the results forthe isoprene-derived intermediates, this indicates that the Criegeeintermediate structure does not have a major impact on thetautomerization reaction energetics.

Conclusions

In summary, electronic structure calculations indicate that theformic acid-catalyzed tautomerization occurs with a free energybarrier that is submerged, or nearly so, for all the Criegee inter-mediates we have examined, including those derived from biogenicsources. This behaviour is in stark contrast to the case of water,which exhibits only a minor catalytic effect on the reaction. Thehigh catalytic activity of monocarboxylic acids is due to the pre-sence of two oxygen functionalities that facilitate the hydrogenatom transfer by converting it to a double hydrogen-shift reaction.Like monocarboxylic acids, the dicarboxylic acids catalyze thereaction to the extent that it becomes barrierless, but the catalyticeffect is somewhat smaller due to the presence of an intramolecularhydrogen bond that inhibits the hydrogen shift. These calculationssuggest the possibility of a facile bimolecular channel in thepresence of carboxylic acids for the decay of Criegee intermediatesto vinyl hydroperoxides and, potentially, hydroxyl radical, whichmay have significant implications for ozonolysis including in thecontext of tropospheric chemistry.

This work is funded by NIFA/USDA grant no. 2011-10006-30362. We thank Dr Michael Lundin and Dr Andrew Danby formany useful discussions.

Notes and references

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Fig. 2 Calculated free-energy barriers for the uncatalyzed and catalyzedCriegee intermediate tautomerization studied in the present work (298.15 K,1 atm, kcal mol�1). Here CI2C, CI3Cl and CI3Cb refer to syn-CH3CHOO, syn-CH3CH2CHOO and (CH3)2COO, respectively.

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